When does 0 × 0 =1?

Mathematics
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Factorials are the product of a given number and all the factors less than that number. The notation n! is used to express this idea. For example, 5! (five factorial) is 5 × 4 × 3 × 2 × 1 = 120. For completeness, 0! is assigned the value 1, so 0 × 0 = 1.